The present application is based on japanese priorty application No. 2000-301466 filed Sep. 29, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to magnetic storage of information and more particularly to a magnetic storage medium for high-density magnetic recording and a magnetic storage device that uses such a magnetic storage medium.
With the progress of information processing technology, the demand for magnetic storage devices having higher recording density is increasing. This demand for increased recording density is particularly acute in a magnetic disk device called hard disk drive that uses a rigid magnetic disk for storing information. Thus, intensive efforts are being made for increasing the recording density of magnetic storage media, and various proposals have been made so far.
Meanwhile, there is a requirement in such high-density magnetic storage media in that a magnetic signal can be reproduced therefrom with low medium noise. Further, the high-density magnetic storage media are required to have a high thermal stability.
As noted before, considerable progress have been made with regard to the improvement of recording density as far as the art of horizontal or lengthwise magnetic recording is concerned. These progresses include development of low-noise magnetic media and high-sensitivity magnetic head such as GMR (giant magneto-resistive) head or spin-valve head.
A typical high-density magnetic recording medium includes a foundation layer formed on a substrate and a magnetic layer is provided on the foundation layer as a recording layer, wherein the magnetic layer is generally formed of a Co alloy layer while the foundation layer may be formed of a Cr layer or a Cr-alloy layer.
Various proposals have been made for reducing the medium noise of high-density magnetic storage media. For example, Okamoto, et al., xe2x80x9cRigid Disk Medium for 5 Gbit/in2 Recording,xe2x80x9d AB-3, Intermag. ""96 Digest, describes an approach that achieves the reduction of the medium noise by way of using a CrMo alloy for the foundation layer. By using a CrMo alloy in the foundation layer, it becomes possible to reduce the thickness of the magnetic layer, while the use of such a thin magnetic layer enables a reduction of particle size and particle size variation in the magnetic layer.
Further, the U.S. Pat. No. 5,693,426 describes the use of a foundation layer formed of NiAl. Further, Hosoe, et al., xe2x80x9cExperimental Study of Thermal Decay in High-Density Magnetic Recording Media,xe2x80x9d IEEE Trans. Magn. vol. 33, 1528, 1997, describes the use of a CrTi alloy for the foundation layer of the magnetic storage media for reducing the medium noise.
The composition of the foundation layer noted above is effective for facilitating in-plane alignment of crystal orientation in the magnetic layer, while such an improvement of crystal orientation in the magnetic layer has an effect of increasing the remnant magnetization and thermal stability of a magnetized recording bit. Furthermore, a decrease of the thickness of the magnetic layer contributes to the improvement of resolution at the time of reading.
Further, investigations are made for reducing the width of the transition region between the recording bits, and further for reducing exchange coupling between the magnetic particles in the magnetic layer by causing segregation of Cr to the grain boundary of the CoCr alloy crystals.
On the other hand, there is a tendency that the thermal stability of the magnetic recording dots formed in the magnetic layer is degraded with decreasing particle size of the magnetic crystals in the magnetic layer, as such a decrease of the particle size facilitates mutual isolation of the magnetic crystals in the magnetic layer. In view of the fact that the demagnetization effect, which is caused in association with the formation of magnetic dots in the magnetic layer, increases with increasing linear density of the magnetic recording dots on the magnetic storage medium, the high-density magnetic storage medium having such ultrafine magnetic particles in the magnetic layer becomes extremely susceptible to thermal agitation.
According to Lu, et al., xe2x80x9cThermal Instability at 10 Gbit/in2 Magnetic Recording,xe2x80x9d IEEE Trans. Magn. vol. 30, pp. 4230, 1994, it was demonstrated, by way of micro-magnetic simulation, that a magnetic storage medium containing magnetic particles having a diameter of 10 nm in the magnetic layer experiences an extensive thermal decay when a signal is recorded with a linear recording density of 400 kfci (fci: flux-change per inch), provided that the anisotropy constant Ku is set so as to satisfy the relationship Kuxe2x80xa2V/kBxe2x80xa2Txcx9c60 for suppressing the exchange coupling between the magnetic particles. Here, Ku is a constant representing the magnetic anisotropy, V represents an average mass of the magnetic particles, kB represents the Boltzmann""s constant, and T represents the temperature. The foregoing quantity Kuxe2x80xa2V/kBxe2x80xa2T is also called thermal stability coefficient.
Abarra et al., xe2x80x9cThermal Stability of Narrow Track Bits in a 5 Gbit/in2 Medium,xe2x80x9d IEEE Trans. Magn. vol. 33, pp. 2995, 1997, on the other hand, reports that the existence of inter-particle exchange interaction improves the thermal stability of the magnetic dots, based on MFM (magnetic force microscopic) analysis of a CoCrPtTa/CrMo medium designed for a recording density of 5 Gbit/in2, for the case the magnetic layer is recorded with a linear density of 200 kfci.
When the linear recording density exceeds the foregoing value of 200 kfci, on the other hand, it was indicated that suppressing of the inter-particle magnetic coupling is necessary. One solution to deal with this problem would be to increase the magnetic anisotropy of the magnetic layer. However, this approach raises the problem that the magnetic head is subjected to excessive load at the time of writing of information.
It is known that the coercive force of a thermally unstable magnetic medium increases rapidly with decrease of the switching time. This is reported for a magnetic tape medium by Ho et al., xe2x80x9cHigh Speed Switching in Magnetic Recording Media,xe2x80x9d J. Magn. Mang. Mater. vol. 155, pp. 6, 1999, and for a magnetic disk medium by Richter, J. H., xe2x80x9cDynamic Coercivity Effect in Thin Film Media,xe2x80x9d IEEE Trans. Magn. vol. 34, pp. 1540, 1999. Such a dynamic change of the coercive force causes an adversary effect at the time of high-speed writing in that the magnitude of the magnetic field induced by the magnetic head for writing information has to be increased with decreasing switching time.
Meanwhile, there is a proposal to improve the thermal stability of magnetic storage medium by applying a suitable texture processing to the substrate located underneath the magnetic layer. By applying such a texture processing, the alignment of magnetic crystals in the magnetic layer is improved. For example, Akimoto, et al., xe2x80x9cMagnetic Relaxation in Thin Film Media as a Function of Orientation,xe2x80x9d J. Magn. Magn. Mater., 1999, discovered, based on micromagnetic simulation, that the effective value of the term Kuxe2x80xa2V/kB increases in response to slight increase of the crystal alignment in the magnetic layer. Based on this discovery, Abarra, et al. could successfully reduce the time-dependence of the coercive force and improve the overwrite performance of the magnetic medium, as is reported in Abarra, et al., xe2x80x9cThe Effect of Orientation Ratio on the Dynamic Coercivity of Media for  greater than 15 Gbit/in2 Recording,xe2x80x9d EB-02, Intermag. ""99, Korea.
Further, there is a proposal of the use of a keeper layer for improving the thermal stability of a magnetic storage medium further.
A keeper layer comprises a soft magnetic layer provided parallel to the magnetic layer above or below the magnetic layer, wherein the soft magnetic layer functions to reduce the demagnetization field of the magnetic bits recorded in the magnetic layer. Typically, a Cr magnetic insulation layer is interposed further between the magnetic layer and the soft magnetic layer.
This approach, however, has a problem in that the magnetic decoupling between the particles in the magnetic layer cannot be achieved due to the existence of continuous exchange coupling between the magnetic layer and the soft magnetic layer and the resultant increase of the medium noise.
In order to eliminate the foregoing problem, the inventor of the present invention has proposed, in a related art application of the present invention, a magnetic storage medium comprising first and second magnetic layers exchange coupled with each other. In the magnetic storage medium of the related art, the second magnetic layer forms an exchange coupled structure together with a non-magnetic coupling layer provided thereon, and the first magnetic layer is provided in exchange coupling with the second magnetic layer via the non-magnetic coupling layer. Thereby, both magnetic layers are magnetized in an anti-parallel relationship.
According to the foregoing proposal, it was demonstrated that the thermal stability of the magnetic bits recorded on the magnetic layer is improved significantly and the medium noise is reduced at the same time. Thus, the magnetic storage medium of the foregoing related art achieves the reliable high-density magnetic recording with minimized medium noise. More specifically, the magnetic storage medium of the foregoing related art has an advantageous feature, associated with the anti-parallel relationship of magnetization between the first and second magnetic layers and partial cancellation of the magnetization caused as a result of such a construction, in that the effective size of the magnetic particles can be increased without causing substantial effect on the resolution of the magnetic storage medium. By increasing the effective size, and hence the effective volume, of the magnetic particles in the magnetic layer, the thermal stability of the magnetic storage medium is improved substantially.
In the magnetic storage medium of the foregoing related art, a parallel relationship appears in the magnetization of the first and second magnetic layers when an external magnetic field is applied to the magnetic storage medium and that the initial anti-parallel relationship is restored when the external magnetic field is removed. For this to occur, it is necessary that one of the first and second magnetic layers undergoes magnetic reversal in response to the reduction of the external magnetic field. It should be noted that the magnetic storage medium of the related art achieves the desired thermal stability by increasing the overall or total thickness of the magnetic layers therein, while causing magnetic reversal in some of the magnetic layers.
When one or more of the magnetic layers supposedly causing the magnetic reversal in the magnetic storage medium have failed to cause the necessary magnetic reversal, a medium noise is induced and the medium noise thus induced causes a difficulty in the designing of the magnetic storage medium or the magnetic head that cooperates with the magnetic medium.
General solution for improving a magnetic storage medium with regard to thermal stability would be:
(1) to increase the volume of the particles by increasing the thickness of the magnetic layer; or
(2) increase the magnetic anisotropy constant Ku.
However, any of these conventional approaches applied to the magnetic layer that is supposed to cause the magnetic reversal in the magnetic storage medium would invite an increase in the coercive force, and the magnetic reversal would become difficult.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic storage medium wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a magnetic storage medium capable of reducing medium noise and simultaneously improving thermal stability of magnetic recording made thereon.
Another object of the present invention is to provide a magnetic storage medium comprising:
at least first and second magnetic layers provided on a support substrate, said first and second magnetic layers having respective first and second magnetizations in an anti-parallel relationship in a state no substantial writing magnetic field is applied to said magnetic storage medium,
said first and second magnetizations being in a parallel relationship when a writing magnetic field is applied to said magnetic storage medium,
said parallel relationship between said first and second magnetizations being changed to said anti-parallel relationship with diminishing of said writing magnetic field as a result of an action of a reversing magnetic field that dominates before said writing magnetic field is diminished and becomes zero.
According to the present invention, the reversing magnetic field acting upon the second magnetic layer that is supposed to have an anti-parallel magnetization with regard to said first magnetic layer dominates with decrease of the writing magnetic field in the opposite direction as the writing magnetic field. As a result, the first and second magnetic layers are in the anti-parallel relationship when the writing magnetic field is eliminated, and the thermal stability of the recorded magnetic bit is improved substantially. Further, the medium noise is reduced and highly reliability is achieved for the magnetic recording by using the magnetic storage medium of the present invention.